Varies an awful lot between countries. Ive done my last IR for 2 month last autumn and tried it out. In general: the further north, the more chance, the further south, the less chance.

In many countries there is also more stable WiFi in main stations-f.e. in FR they now often even have special counters/seats for that in the waiting area-and then you discover it will only take FR numbers!

In DE/DB usually very good on the ICE/IC-local trains very variable. AT/OeBB-good in the RailJet and newer local trains. CH/SBB; NOT. BE/NMBS: only with a very cumbersome registration, did not try that. FR: can do in most TGV-but you have to discover the sneaky way to get around the ´only for FR nrs!

But if you have a plan with a fone-you can easily upgrade or buy extra MB when on the road.